Suppressor T-cells (STC) of thymus, spleen and lymph nodes will be investigated for physical properties (adherence to glass, size, density) and ultrastructural characteristics. Explanations will be sought for their marked increase in animals treated with steroids, thymectomized as adults, or given a large dose of antigen, and an attempt made to relate STC in the thymus to those in the periphery. Sensitized T-cells in lymph nodes draining a footpad inoculation site, will be separated by physical means from STC and from mitogen- reactive cells in the same population and characterized functionally and ultrastructurally. New lymphokines, active on e.g. vascular endothelium, will be sought and characterized. The newly synthesized DNA, which is excreted by lymphocytes during their response to standard mitogens and antigens, will be characterized physico-chemically, and examined for informational content and for its uptake by other cells. Continuous cell lines with differentiated T-cell functions will be developed and studied in an attempt to learn more about genetic control of individual functions, notably antigenic markers of the cell membrane and lymphokine production.